Opentelemetry_client_ocurl.cleanup does not flush all traces before shutting down properly

[ajbt200128]

When using the Opentelemetry_client_ocurl collector, if we call cleanup on the backend, not all traces are flushed.

One can reproduce this by setting up the backend, start tracing, send many of them, and then call cleanup right before exiting. When I call Backend.tick () first, all traces will appear everytime. We should probably just call Backend.tick in the cleanup function

[ajbt200128]

Related #68 for why one might want to shut down a backend early

[c-cube]

#74 should fix this I think.

[ajbt200128]

Thanks!